Fuel cell systems conveniently operate at elevated pressures. Since the fuel cell system develops substantial heat, the turbo compressor arrangement is conveniently used. The compressor compresses the air feeding the components of the fuel cell system using the energy in the gas products leaving the fuel cell system at elevated pressure as well as high temperature as these exhaust products then pass through the turbine portion of the turbo compressor.
At certain loads on the system there may not be sufficient heat generated to operate the turbo compressor at the proper level. Accordingly, an auxiliary burner is supplied which receives air from the compressor and which fires fuel to supplement the energy from the fuel cell system.
Such an auxiliary burner uses a primary burner which is modulated as required to maintain appropriate compressor discharge pressure. It also includes a pilot which is burning continuously and assures ignition. For safety reasons such auxiliary burner also includes a flame sensor to detect the presence of flame. If the flame is extinguished for any reason, this fuel sensor sends a signal to a valve in the fuel supply to stop fuel to the auxiliary burner.
It is common for auxiliary power lines in plants to experience occasional intermittent voltage loss lasting 1 to 5 seconds, and occasional longer intermittent voltage loss lasting up to 15 seconds. These are related frequently to multiple tries at auxiliary power line breaker reclosure after certain conditions such as lightening strikes. Since the fuel is being compressed for a fuel cell power plant, such loss of power can result in a rapid loss of fuel pressure to the fuel cell and the auxiliary burner. The rapid decrease in fuel supply or complete loss of fuel to the auxiliary burner will result in a flameout which is sensed by the flame sensor cutting off fuel to the auxiliary burner.
Since combustibles may exist within the furnace chamber of the burner, it is not safe to immediately restart the burner, but the unit must first be purged and then restarted. This longer term outage of the auxiliary burner leads to a shutdown of the entire plant.
It therefore is advantageous to maintain operation of the auxiliary burner, or at least the pilot therein during these brief power losses.